Secondary oxide mineral nodules occur in the ferruginous, ferruginous
bauxitic, and bauxitic soils of the Hawaiian Islands. The concentration of these
oxide nodule aggregates which are larger than 2 mm ranges from 5 per cent in some
ferruginous soils to as high as 85 per cent in the ferruginous bauxitic soils of the
Halii family. The nodules are formed by the process of induration in which the
hydrated amorphous hydroxide and oxides are dehydrated to their crystalline oxide
form. The resulting indurated oxide nodule becomes an independent unit in the soil
system with corresponding loss of effective surface even though the nodule may
contain friable clay material in its interior. The typical nodule produced in the ferruginous
bauxitic soil has a dense iron oxide layer (hematite and maghemite) and,
in the interior, soft to hard, lighter-colored material which contains gibbsite.